Three-dimensional coordinate information representing neuronal processes is obtained utilizing a video version of the computer microscope. Usually specimens are prepared by the rapid Golgi method although the measuring system has the capability of accurately reconstructing and analyzing most biological structures which are histologically prepared and preserved on microscope slides. The opportunity is also presented to serially reconstruct electron micrographs. Neurons from the hippocampal and related areas of the human immature brain (18 to 33 weeks gestational age) have been measured and analyzed according to their branching characteristics. Statistical parameters representing the branching characteristics of neuronal processes (e.g. branching/fission angles, polar angle and radial separation distributions of endpoints, etc.) are obtained and attempts are made to correlate any observed deviant patterns with specific forms of profound mental retardation (e.g., Down's syndrome, chromosomal abnormalities, etc.). Attempts are also in progress to quantitatively analyze and classify dendritic spines in regard to their density, distribution and topological characteristics.